This allows programs that need to have repeatable results to not have to come up with their own seed generating mechanism.
Instead,
they can use srand() and somehow stash the return for future use.
Typical is a test program which has too many combinations to test comprehensively in the time available to it each run.
It can test a random subset each time,
and should there be a failure,
log the seed used for that run so that it can later be used to reproduce the exact results.

\N{},
charnames::vianame,
charnames::viacode now know about every character in Unicode.
Previously,
they didn't know about the Hangul syllables nor a number of CJK (Chinese/Japanese/Korean) characters.

Only stable releases (5.10.x,
5.12.x,
5.14.x,
...) guarantee binary compatibility with each other,
while blead releases (5.13.x,
5.15.x,
...) often break this compatibility.
However,
prior to perl 5.13.4,
all blead releases had the same PERL_API_REVISION,
PERL_API_VERSION,
and PERL_API_SUBVERSION,
effectively declaring them as binary compatible,
which they weren't.
From now on,
blead releases will have a PERL_API_SUBVERSION equal to their PERL_SUBVERSION,
explicitly marking them as incompatible with each other.

Maintenance releases of stable perl versions will continue to make no intentionally incompatible API changes.

When perl's API changes in incompatible ways (which usually happens between every major release),
XS modules compiled for previous versions of perl will not work anymore.
They will need to be recompiled against the new perl.

In order to ensure that modules are recompiled,
and to prevent users from accidentally loading modules compiled for old perls into newer ones,
the XS_APIVERSION_BOOTCHECK macro has been added.
That macro,
which is called when loading every newly compiled extension,
compares the API version of the running perl with the version a module has been compiled for and raises an exception if they don't match.

After assignment to $[ has been deprecated and started to give warnings in perl version 5.12.0, this version of perl also starts to emit a warning when assigning to $[ in list context. This fixes an oversight in 5.12.0.

When doing a lot of string appending, perl could end up allocating a lot more memory than needed in a very inefficient way, if perl was configured to use the system's malloc implementation instead of its own.

sv_grow, which is what's being used to allocate more memory if necessary when appending to a string, has now been taught how to round up the memory it requests to a certain geometric progression, making it much faster on certain platforms and configurations. On Win32, it's now about 100 times faster.

For weak references, the common case of just a single weak reference per referent has been optimised to reduce the storage required. In this case it saves the equivalent of one small perl array per referent.

XPV, XPVIV, and XPVNV now only allocate the parts of the SV body they actually use, saving some space.

Carp now detects incomplete caller() overrides and avoids using bogus @DB::args. To provide backtraces, Carp relies on particular behaviour of the caller built-in. Carp now detects if other code has overridden this with an incomplete implementation, and modifies its backtrace accordingly. Previously incomplete overrides would cause incorrect values in backtraces (best case), or obscure fatal errors (worst case)

Besides listing the updated core modules of this release, it also stops listing the Filespec module. That module never existed in core. The scripts generating Module::CoreList confused it with VMS::Filespec, which actually is a core module, since the time of perl 5.8.7.

When using old 32-bit compilers, the define _USE_32BIT_TIME_T will now be set in $Config{ccflags}. This improves portability when compiling XS extensions using new compilers, but for a perl compiled with old 32-bit compilers.

The PERL_STATIC_INLINE define has been added to provide the best-guess incantation to use for static inline functions, if the C compiler supports C99-style static inline. If it doesn't, it'll give a plain static.

HAS_STATIC_INLINE can be used to check if the compiler actually supports inline functions.

substr(), pos(), keys(), and vec() could, when used in combination with lvalues, result in leaking the scalar value they operate on, and cause its destruction to happen too late. This has now been fixed.

Building with PERL_GLOBAL_STRUCT, which has been broken accidentally in 5.13.3, now works again.

If you find what you think is a bug, you might check the articles recently posted to the comp.lang.perl.misc newsgroup and the perl bug database at http://rt.perl.org/perlbug/ . There may also be information at http://www.perl.org/ , the Perl Home Page.

If you believe you have an unreported bug, please run the perlbug program included with your release. Be sure to trim your bug down to a tiny but sufficient test case. Your bug report, along with the output of perl -V, will be sent off to perlbug@perl.org to be analysed by the Perl porting team.

If the bug you are reporting has security implications, which make it inappropriate to send to a publicly archived mailing list, then please send it to perl5-security-report@perl.org. This points to a closed subscription unarchived mailing list, which includes all the core committers, who be able to help assess the impact of issues, figure out a resolution, and help co-ordinate the release of patches to mitigate or fix the problem across all platforms on which Perl is supported. Please only use this address for security issues in the Perl core, not for modules independently distributed on CPAN.